1. Field of the Invention
The present invention relates to a solid state device, and particularly, to a solid state device which is sealed with a sealing material made of low melting point glass.
The present invention also relates to a light-emitting element made of a semiconductor material and a light-emitting device using the light-emitting element, and particularly, to a light-emitting element and a light-emitting device which are flip-chip mounted.
Herein, a solid state device includes various optical devices or elements such as a light-emitting device or element, a light-receiving device or element, and a solar cell. Also, herein, a light-emitting element includes an LED element, and a light-emitting device includes an LED.
2. Description of the Related Art
Conventionally, there are solid state devices with a solid state component such as a light-emitting diode (LED), or the like, sealed with a light-transmitting resin material such as epoxy resin, or the like. It is known that in such solid state devices, light-transmitting resin is degraded by light. Particularly, in the case of use of group III nitride-based compound semiconductor light-emitting elements which emit short-wavelength light, light-transmitting resin around the elements is caused to turn yellow by high-energy light emitted from the elements and heat produced by the elements themselves, which often unnegligibly degrades efficiency of deriving light.
To prevent degradation of such sealing materials, there have been proposed light-emitting devices using low melting point glass as sealing material (See Japanese patent application laid-open Nos. 8-102553, and 11-177129, for example).
The light-emitting device of Japanese patent application laid-open No. 8-102553 is constructed by covering an LED element, wire bonding portions, and upper ends of leads, with a transparent sealing material made of low melting point glass. In the low melting point glass, there is used glass whose melting point is 130-350° C. by adding thereto selenium, thallium, arsenic, sulfur, etc., for example. In this case, there is used low melting point glass whose melting point is preferably 200° C. or less, more preferably 150° C. or less.
The light-emitting device of Japanese patent application laid-open No. 8-102553 uses a transparent sealing material made of low melting point glass, thereby obviating the problem with optical degradation with time of light-transmitting resin material due to ultraviolet rays.
On the other hand, the light-emitting device of Japanese patent application laid-open No. 11-177129 uses, as sealing material for covering its LED element, low melting point glass whose refractive index is on the order of 2 close to the refractive index of GaN-based LED elements, the order of 2.3.
The light-emitting device of Japanese patent application laid-open No. 11-177129 seals its LED element with low melting point glass whose refractive index is close to the refractive index of GaN-based LED elements, thereby lessening light totally reflected off the interface between the LED element and the low melting point glass, while increasing light radiated outwardly from the LED element and passed into the low melting point glass. As a result, the outward radiation efficiency becomes higher than that of conventional light-emitting devices with an LED element sealed with epoxy resin,
According to conventional solid state devices, however, because of high viscosity of conventional low melting point glass in a practical sealing temperature range, it is impossible to realize a solid state device having the sufficient sealing property when using a sealing material made of low melting point glass.
Conventionally, on the other hand, flip-chip mounting is known in which an LED element is electrically connected to a wiring pattern of a printed wiring board or the like through a stud bump of Au or the like.
In flip-chip mounting, a cathode and an anode on the LED element are connected to a wiring pattern via a stud bump, thus allowing mounting of the LED element without using pad electrodes and wires. In addition, light is radiated from the surface opposite the mounting surface, which thus allows excellent light-radiating performance without pad electrodes and wires blocking off light.
The above-mentioned flip-chip mounting requires arrangement of stud bumps according to the number of cathodes and anodes, and stable arrangement of the LED element requires 3 or more stud bumps. This requires time-consuming and costly bump formation. Particularly, in large-size LED elements, a plurality of stud bumps are arranged for multipoint joining, which can therefore be more significantly time-consuming and costly.
As a means for obviating time-consuming labor in such bump formation, there is a bump formation method in which a bump for mounting components is formed on the surface of a printed wiring board by plating (See Japanese patent application laid-open No. 2002-9427).
According to a bump formation method described in Japanese patent application laid-open No. 2002-9427, a resist film is formed by applying resist to the surface of a printed wiring board, using a spin coater or a printing method. The resist film formation is followed by light exposure using a mask having a mask window matching a desired position and shape of a bump. Next, an exposed portion is developed and dissolved by an immersion or spray method used in typical photoresist development to form an opening, followed by plating the opening and subsequently dissolving and removing the resist film, thereby forming a plurality of bumps for mounting components on the printed wiring board.
In the above-mentioned bump formation method, however, because the bumps for mounting components have to be formed by photolithography, the number of fabrication steps increases, which results in a high cost.
Further, it is required that the bumps for mounting components have high shape accuracy matching electrode shape of the light-emitting element, and that the light-emitting element be positioned accurately relative to the bumps for mounting components when mounting the light-emitting element.